1. Field of the Invention
The present invention relates to cooling systems for semiconductors generally and more particularly to cooling systems for power semiconductors of a battery powered vehicle.
2. Description of the Prior Art
Battery powered vehicles such as electric lift trucks utilize power semiconductors such as SCR's (silicon controlled rectifiers) to control the application of battery power to the vehicle drive motor and hence to control the speed of the vehicle. The current through the semiconductors generate heat which must be dissipated to insure adequate life and trouble-free performance of the semiconductor and the vehicle generally and the vehicle drive system in particular. To dissipate this heat generated by the power semiconductors various cooling systems have been adopted.
A commonly accepted method of cooling the vehicle power semiconductors is to mount the semiconductors on solid heat sinks. These heat sinks are manufactured from materials such as copper, brass, or aluminum, which have a high heat transfer coefficient. The semiconductors are usually mounted in direct contact with the heat sinks and conduct heat to the heat sinks from which the heat is dissipated by natural convection from the surface of the heat sink. To increase the amount of heat convected from the heat sink the heat sink is sometimes formed to provide a finned external surface. This opens more area for convection heat transfer.
Nevertheless, whether the heat sink is finned or not, its placement in a relatively cool and open area of the vehicle is needed to assure proper heat dissipation. This placement requirement does not necessarily provide a compact drive control package since the cool area may be remote from the vehicle motor which is being controlled.
Since the vehicle is driven at different speeds, the power controlling semiconductor must handle different degrees of power transmission. A higher power level transmission requires a longer ON time for the semiconductor and consequently the semiconductor if self-heated to a greater degree and more heat must be removed therefrom to insure proper operation and life. A heat sink that is adequate for normal vehicle duty cycle heat dissipation may be inadequate to handle extended semiconductor controlled speed operation of the vehicle. Under such extended semiconductor controlled speed operation the semiconductor can overheat and fail. To prevent such semiconductor damage, present electric vehicles provide a thermistor which senses the temperature of the heat sink and cuts back the power to the semiconductor upon detecting such an overheating condition. To accomplish this a thermistor having a positive temperature coefficient electrically controls the gate frequency of the SCR and thereby provides a diminishing of the gate control frequency as the heat sink approaches danger levels. One undesirable feature of this system is that the vehicle does not travel at the speed dictated by the accelerator pedal position.
Liquid cooling of the heat sink would provide adequate heat transfer from the normal heat sink to allow the vehicle to operate under various extended speed conditions. As a concept, liquid cooling of semiconductors is disclosed in U.S. Pat. No. 3,400,543 issued to P. G. Ross. However, the adaption of a liquid cooling system to cool the power semiconductors of a battery driven electric vehicle presents problems which have not been solved to date by the prior art. Specifically, since the power source of an electric vehicle is D.C., the pump circulating the cooling fluid would have to operate from a D.C. power source to provide an economically feasible system. The use of available A.C. pump motors would require the use of expensive circuitry to convert the D.C. power to A.C. power. On the other hand, available D.C. pump motors are expensive or toublesome requiring intermittent brush replacement. A D.C. pump and motor would also require more space. As such the D.C. motor driven pump is also unacceptable as a component of a liquid cooling system for cooling power semiconductors of a battery driven vehicle.